This was a convention used for the Census 2000 ZCTAs, because the requirements for delineation stated that all census blocks must receive a ZCTA code.

The three-digit ZCTA code and an "XX" suffix were applied to large land areas (generally larger than 25 square miles) where there was insufficient information to determine the five-digit ZIP Code. These were generally rural areas with little settlement; for example, parks, forests, deserts, and mountainous areas. Smaller areas were usually absorbed into adjacent five-digit ZCTAs, but for larger areas, we chose not to overextend the surrounding five-digit ZCTAs.

The three-digit ZCTA code and an "HH" suffix were applied to water features. Because large water features may border many five-digit ZCTAs, it would be difficult to divide the water features and assign the parts to the surrounding five-digit ZCTAs. Adding these water features to land-based ZCTAs would also have greatly distorted the size and shape of the land ZCTAs, particularly in coastal areas.

For the 2010 and 2020 Census, there was no requirement for wall-to-wall coverage, so there was no need for "XX" or "HH" ZCTAs. Only five-digit ZCTAs were produced, and large unpopulated areas were excluded from the delineations.